The invention relates to a process and an apparatus for the continuous production of a thin metal strip, in particular a steel hot strip, directly from a metal melt and with a strip cast thickness of <10 mm after a roll-casting process using a roll-casting device.
In particular, the invention relates to a process and an apparatus for producing a hot-rolled steel strip with a strip cast thickness of <6 mm. The hot strip thickness when the hot strip is stored following the rolling deformation is between 0.3 and 4 mm.
The proposed roll-casting processes on which the invention is based encompass all types of casting processes in which metal melt is solidified on the lateral surface of a casting roll so as to continuously form a metal strip. Both the single-roll casting process using a single-roll casting device and the vertical or horizontal two-roll casting process using a two-roll casting device are suitable for implementing the invention. It is also appropriate for the axes of the two interacting casting rolls to be arranged in a plane that is inclined obliquely with respect to the horizontal in order to implement the process according to the invention.
In a vertical two-roll casting process, metal melt is introduced into a melt space which is laterally delimited by two rotating casting rolls and associated side plates, with the axes of rotation of the casting rolls lying substantially in one horizontal plane. The two casting rolls with the associated side plates, including the necessary actuating and control devices, in this case form the core component of the two-roll casting device. The metal melt solidifies continuously at the lateral surfaces of the rotating, internally cooled casting rolls and forms strand shells which are moved with the lateral surfaces. In the narrowest cross section between the two casting rolls, the two strand shells are joined to form an at least substantially fully solidified metal strip. The cast metal strip is discharged at casting speed between the casting rolls and then fed for in-line thickness reduction in a rolling installation. Then, the rolled hot strip is fed to a storage device, in which it is stored. This process is preferably suitable for the production of steel strip, but it is also possible for metal strips made from aluminium or an aluminium alloy to be produced in this way. The basic principles of processes and installations of this type are already known, for example from WO 01/94049 or WO 03/035291.
To ensure further processing without any problems, flatness tolerances, which are in some cases defined in standards and in other cases are requested by customers according to the intended further processing, have to be maintained by the rolled hot strip. Experience gained in the production of hot-rolled steel strip has shown that it is very difficult to satisfy these requirements when using the two-roll casting process on a corresponding casting installation.
Standard values for the flatness of thin hot strip are defined in standards (e.g. DIN 10051) and for rolled hot strip have values of from 20 to 30 I units for the thickness range described in the introduction.
One major cause of difficulties in achieving standard flatness values results from the high production speed with the production process selected for the cast intermediate product. The metal strip is produced in a process with extremely high solidification rates directly in a format with extreme width/thickness ratios, which although eliminating a large number of roll passes with a view to achieving the desired hot strip final thickness, means that width-independent, uniform convective heat transfer or liquid metal temperature at the solidification front (when forming the strand shells) are only possible to a limited extent, on account of the highly turbulent flow conditions in the metal bath. This results in a temperature/width profile on the cast metal strip when it emerges from the casting nip between the casting rolls which is subject to fluctuations of up to 100% and above, based on the supercooling with respect to the equilibrium solidus temperature, so that internal stress conditions and creep properties which cause unevenness in the cast strip are present. Unevenness which lies outside the hot strip standard is produced even if the fluctuation is only in a range from 30-40%.
The in-line rolling of a cast metal strip can also contribute to the formation of further unevenness if the strip inlet temperature (temperature at which the metal strip enters the rolling stand) is relatively uneven over the width of the metal strip or the inlet strip profile is unknown or fluctuates. This results in variable deformation properties in the roll nip as a result of different spring properties or roll nip profiles transversely with respect to the rolling direction.
When it first enters a rolling stand, the cast metal strip has an entry microstructure with a cast structure which with a low reduction per pass is converted into a more fine-grained rolled microstructure, in order to achieve the materials properties which are favourable for the respective further processing steps. At the same time, the starting thickness upstream of the rolling stand is less than 10 mm, preferably less than 6 mm. At the low starting thicknesses preferred, it is not possible to influence the relative strip profile without any flatness defects. Furthermore, the high roughness of the metal strip, caused by the casting operation and by any scaling, leads to a high level of wear to the working rolls. These wear phenomena on the working rolls occur to an increased extent in the region of the strip edges and lead to defects in the strip profile. In this context, apart from the strip thickness and the temperature level, the wear phenomena are also influenced to a considerable extent by the strip material, the strip profile and the thermal profile.